1. Technical Field
The technical field relates to an imaging apparatus, such as a digital still camera or a digital video camera, having drive unit that can drive a focus lens.
2. Related Art
A conventional digital camera includes a motor (actuator) for driving a zoom lens, a focus lens, a diaphragm, or the like (see, for example, JP-A-2008-92619). For the digital camera described above, various proposals are provided, such as improvement of drive noises of the motor, the maximum operation speed, precision at a stop position, power consumption, or the like.
In order to save power consumption of the digital still camera, it can be considered not to supply power to a motor that is not being driven. However, when no power is supplied to a motor mounted in a normal digital camera, the motor cannot allow a rotor to stop at a desired position accurately, so that a lens or the like cannot be controlled at a desired position. This will be detailed below with a stepping motor taken as an example.
FIGS. 12A and 12B are sectional views of a stepping motor. The stepping motor includes a rotor 10 made of a magnet and a stator 20 provided with an electromagnet. The stepping motor rotates the rotor 10 by attraction force caused between the magnet of the rotor 10 and magnetic field which is generated by an electric current flowing through a stator winding 22.
A stepping motor that drives a movable lens with a micro-step drive does not turn on and off a conduction of electric current to the stator winding 22 simply, but controls finely the ratio of the electric current flowing through the adjacent two windings, so that the position of the rotor 10 is controlled finely. Therefore, when no current is supplied to the stator winding 22 of the stepping motor, a magnetic field is not produced in the stator winding 22, so that the rotor 10 cannot be maintained at a desired position. For example, if power supply is stopped on the condition that the rotor 10 is located at the desired position by supplying the power to the stator winding 22 (FIG. 12A), the rotor 10 moves and stops at the position shifted from the desired position (FIG. 12B). This operation will specifically be described below. The electromagnet portion 24 that generates a magnetic field by supplying a current to the windings of the stator 22 is magnetized by the magnetic force of the rotor 10 when the supply of the current to the stator winding 22 is stopped. Therefore, attraction force is generated between the electromagnet portion 24 and the rotor (magnet) 10 so that the rotor 10 moves to a specific position (stable point). Specifically, as shown in FIG. 12A, the adjacent electromagnet portions 24 that are magnetized with a predetermined ratio are uniformly magnetized by the magnetic force of the magnet of the rotor 10 when the supply of the current to the stator winding 22 is stopped. Accordingly, the rotor 10 stops at the middle position between the adjacent electromagnet portions 24 as shown in FIG. 12B. This stop position is the stable point.
FIGS. 13A and 13B are views showing the configuration of a driver that drives a focus lens by the stepping motor. As shown in FIG. 13A, when the power is supplied to the stepping motor 33 that drives the focus lens 31 (when excitation ON), the focus lens 31 is stopped at a desired stop position. On the other hand, when no power is supplied to the stepping motor 33 (when excitation OFF), the focus lens 31 is stopped at a position shifted from the desired stop position as shown in FIG. 13B. Accordingly, for the motor described above, precision in a stop position of the lens is degraded when no power is supplied to the motor.